The present invention relates to a television receiver having a cathode ray tube (CRT) and, more particularly, to a digital convergence correcting apparatus for correcting a distortion of a projection television receiver and a convergence.
In association with the spread of VTR and video disk players, a desire to see and enjoy a powerful image has grown and a wide screen of a television receiver and a home theater have rapidly been realized. Thus, there has widely been used what is called a projection television in which an image reproduced on the fluorescent screen of the CRT display is enlarged projected onto the display screen by a projection optical system such as lens, mirror, and the like to thereby obtain an image of a large screen.
In a projection television, since aspherical plastic lenses, an aspherical projection tube fluorescent screen, and a short projection optical system are used, the color aberration, spherical aberration, coma aberration, astigmatism, and image surface curve are excellent and the sharpness (focus) of the image is good.
However, the magnifications of respective portions of the screen differ and what is called a distortion in which an object figure and its image are not analogous occurs. With respect to the distortion, it is necessary to consider two parameters such as an absolute value and a distortion of a high order. Generally, in a optical design, the aberration and distortion are contradictory and a compromising point between them is used as a design value.
Therefore, when a focusing performance is raised to a limit value, the distortion increases. particularly, the high-order distortion of the distortion increases. Further, in the projection television, three color lights from three projection tubes of R, G, and B are collected from different angles and an image is formed on the screen. Therefore, convergence performance becomes more complicated with an increase in distortion.
A known method of correcting distortion, analog convergence correction, is disclosed in "Color Television Textbook", Nippon Hoso Kyokai, pages 262 to 265; and the like. According to such method, a sawtooth wave and a parabolic wave which are synchronized with a horizontal sync signal and a vertical sync signal are combined and a correction current is allowed to flow through a convergence yoke (hereinafter, abbreviated to CY), thereby correcting the distortion and the convergence.
However, according to such a method, since there is a limitation in the combination of waveforms, the distortion of the screen and the misconvergence cannot be perfectly corrected.
Therefore, digital convergence correcting apparatus is used for storing an arbitrary waveform according to the complicated distortion into a memory and for converting such into an analog waveform, thereby driving the CY.
In the ordinary conventional digital convergence correcting apparatus, as disclosed in JP-A-50-68012 and JP-A-61-12191, by providing one D/A converter for each of the CY to be driven and the convergence coil, the correction is accomplished. In the television receiver which needs a number of CYs and convergence coils, the number of D/A converters is at least equal to the number of channels.
The digital convergence correcting apparatus can correct at a high accuracy the complicated distortions and misconvergence which occur when using the aspherical plastic lenses, aspherical projection tube fluorescent screen, and short projection optical system.
However, to realize the digital processes, the circuit scale increases and the D/A converters equal in number to the number of CYs which are driven are needed. Particularly, in a system such as a projection television or the like using a number of CYs to be driven, there is a problem which arises in that the circuit scale and the cost increase.
On the other hand, there is also a problem such that each of the D/A converters generally generate a glitch as a pulse-like noise in the analog output voltage and an interference is created such that a desired analog waveform cannot be obtained.